1. Field of the Invention
The present invention relates to a method for illuminating in a liquid crystal display device, a back-light assembly for performing the same and a liquid crystal display device using the same, and more particularly, to a method for illuminating a liquid crystal display device and a liquid crystal display device using the same. The method for illuminating a liquid crystal display device enhances the front luminance of the liquid crystal display device, reduces the manufacturing process steps and renders a thin and light liquid crystal display device.
2. Description of the Related Art
Generally, a liquid crystal display device applies a voltage to a specific molecular arrangement of a liquid crystal to convert its molecular arrangement. Then, the liquid crystal display device converts the changes of the optical properties such as birefringence, optical linearity, dichroism and optical scattering characteristics of liquid crystal cells that emits light according to the molecular arrangement, and uses the modulation of the light of the liquid crystal cells.
The liquid crystal display device is largely sorted into a TN (Twisted Nematic) type and a STN (Super-Twisted Nematic) type. The liquid crystal display device is, according to the driving method, sorted into an active matrix display type, which uses a switching device and a TN liquid crystal, and a passive matrix type, which uses an STN liquid crystal.
A distinguishable difference of two types is that the active matrix display type is a TFT-LCD that drives an LCD by using a TFT and that the passive matrix display type does not have such a complicated circuit associated with a transistor because it has no transistor.
Since the liquid crystal display device is a passive light element incapable of emitting light itself, the images are displayed by using a back-light assembly attached to the rear plane of the liquid crystal panel.
Recently, there are several structures developed for contriving the slim and light products so as to have the leading edge of the competitiveness. Especially, the light-weight becomes a more important factor in view of the adoption chiefly to a portable computer, etc.
In such a liquid crystal display device, the role and function of the back-light assembly have been considered as increasingly important, because the dimension, light efficiency and so on differ depending on the back-light assembly structure to affect on overall mechanical/optical characteristics of the liquid crystal display device.
A structure of a general back-light assembly used in the liquid crystal display device is disclosed in U.S. Pat. No. 5,467,208.
FIG. 1 schematically shows the foregoing back-light assembly of the conventional liquid crystal display device. FIG. 2 schematically shows an illuminating process via the back-light assembly of the conventional liquid crystal display device.
Referring to FIGS. 1 and 2, the liquid crystal display device includes a lamp 1 for generating lights, a light guide plate 4 for guiding the lights from the lamp 1, and a lamp housing 2 installed to the side plane of the light guide plate 4 surrounding the lamp 1.
The lamp 1 employs a cold cathode, and the lights generated from the lamp 1 are incident via the side plane of the light guide plate 4.
The lamp housing 2 is formed with a reflection plate 6 on the inner surface thereof to reflect the lights from the lamp 1 toward the side plane of the light guide plate 4, thereby enhancing the efficiency of the lights from the lamp 1.
The light guide plate 4 is formed of a transparent material of plastics such as acrylic resin for forming a panel shape having a sloped lower plane and a horizontal upper plane (alternatively, a sloped upper plane) to allow the lights generated from the lamp 1 to advance toward an LCD panel (not shown) seated on the upper portion after passing through the upper plane of light guide plate 4. Accordingly, the lower plane of light guide plate 4 is formed with various patterns such as a minute dotted pattern printed for changing the advancing direction of the lights from lamp 1 into toward the LCD panel side.
A reflection plate 6 is formed under the lower plane of light guide plate 4, and a diffusion sheet 8, a first prism sheet 10, a second prism sheet 12 and a protecting film 14 are sequentially stacked on the upper plane of light guide plate 4.
The reflection plate 6 again reflects the lights that are not reflected by the minute dotted pattern in the lights which have been generated from the lamp 1 to advance toward the lower plane of the light guide plate 4 toward the upper plane of the light guide plate 4, thereby improving the consistency of the lights transmitted to the upper plane of the light guide plate 4 while decreasing the optical loss of the lights incident to the LCD panel. As described above, the light guide plate 4 and the reflection plate 6 guide the lights generated from the lamp 1 toward the upper plane of the light guide plate 4.
At this time, the lights passing through the upper plane of the light guide plate 4 include the lights outgoing by being to be sloped at various angles as well as the lights outgoing perpendicularly to the upper plane thereof.
The diffusion plate 8 placed between the light guide plate 4 and the first prism sheet 10 scatters the incident lights from the light guide plate 4 to prevent the partial concentration of the lights. Additionally, the diffusion plate 8 decreases the sloped angle of the lights advancing toward the first prism sheet 10 with respect to the first prism sheet 10.
The first prism sheet 10 and the second prism sheet 12 are formed in a manner that triangular bar-shaped prisms are formed to respective upper planes in a regular arrangement, and the prism arrangement of the first prism sheet 10 and that of the second prism sheet 12 are provided to cross each other at a predetermined angle.
The first and second prism sheets 10 and 12 respectively serve for condensing the lights diffused by the diffusion plate 8. By doing so, the vertical incidence with respect to the protecting film 14 of the lights having passed through the first and second prism sheets 10 and 12 becomes most favorable.
For this reason, almost all lights passing through the first and second prism sheets 10 and 12 advance perpendicularly, so that the luminance distribution over the protecting film 14 is uniform.
The protecting film 14 formed on the second prism sheet 12 not only protects the surface of second prism sheet 12 but also diffuses the lights for uniform distribution of lights. The LCD panel 15 is installed on the protecting film 14.
In other words, in the back-light assembly structure of the conventional liquid crystal display device, as shown in FIG. 2, the lights are generated from the lamp to be incident to the LCD panel 15 placed on the protecting film 14 of the back-light assembly via the path provided by the diffusion, condensing, condensing then to diffusion.
However, when applying the illuminating method of the back-light assembly, the above-mentioned conventional liquid crystal display device requires several sheets for condensing/diffusing the lights on/from the upper portion of the light guide plate to therefore complicate the assembling process. Furthermore, the plurality of sheets used for the device may render high probability of puckering to degrade reliability of products and increase the manufacturing cost.
In addition, in the conventional diffusion plate, a plurality of beads are distributed over the front plane and rear plane of the same density to distribute the lights incident from the light guide plate to the rear plane of the diffusion plate.
When constructing the diffusion plate having the aforesaid structure, the precision of condensing function conducted by the first and second prism sheets stacked on the diffusion plate is degraded to lower the front luminance when the lights incident from the lower portion of the diffusion plate are aslant.
Moreover, in the conventional liquid crystal display device, if the protecting film is eliminated to decrease the number of sheets stacked in the back-light assembly, the lights condensed by the first and second prism sheets having the triangularly-shaped section are concentratedly condensed only to the direction that is nearly perpendicular to the plane of the LCD panel to be incident to the LCD panel. The lights condensed as described above cause interference with the pixels of the LCD panel, which in turn incites a moiré phenomenon of producing wave pattern throughout the screen.